Find the difference between two collections in Java 8?

Question

I am trying to make a List of all of the books in one Collection that are not present in another. My problem is that I need to compare based on book ID, so I can't just test to see whether a book in the first is contained in the second, I have to determine whether any book in the second collection has the same ID as a book in the first.

I have the below code to compare two collections of books and filter the first collection:

List<Book> parentBooks = listOfBooks1.stream().filter(book->   !listOfBooks2.contains(book)).collect(Collectors.toList()); 

The code doesn't work correctly because I am comparing the objects themselves. I need to compare the objects based on the bookId instead of the whole book object. How should I change the code so it can do the comparison based on the bookId (book.getId())?

Answer 1

List<Book> books1 = ...; List<Book> books2 = ...; Set<Integer> ids = books2.stream()         .map(Book::getId)         .collect(Collectors.toSet()); List<Book> parentBooks = books1.stream()         .filter(book -> !ids.contains(book.getId()))         .collect(Collectors.toList());